Together with drastic increase of communication demand in recent years, development of a wavelength division multiplexing communication system (WDM system) wherein a plurality of signal lights having wavelengths different from each other are multiplexed so that high capacity transmission may be implemented with one optical fiber is advancing.
In such a wavelength division multiplexing communication system as just described, a tunable laser capable of selecting a desired wavelength at a high speed over a wide wavelength range is needed in order to implement a flexible and an advanced communication system.
In a wavelength division multiplexing (WDM) optical communication system, communication with a high capacity can be implemented by transmitting optical signals having wavelengths different from each other within a wavelength range of the C-band (1,525 to 1,565 nm) or the L-band (1,570 to 1,610 nm) through one optical fiber.
In the WDM communication system, from an aspect of inventory control of a laser to be used as a light source, flexibility of system construction and so forth, a tunable laser which can output light with an arbitrary wavelength in the C-band or the L-band described above, that is, which has a tunable width (a wavelength variable width) of approximately 40 nm, is needed.
For example, there is a tunable laser wherein two wavelength filters having periodic peak wavelengths different from each other are disposed in a cavity and the oscillation wavelength is varied by utilizing a vernier effect (hereinafter referred to as first technique).
For example, also there is a tunable laser wherein a wavelength filter having a periodic peak wavelength such as an etalon and a tunable filter capable of varying the transmission wavelength or the reflection wavelength over a wide band are disposed in a cavity and the oscillation wavelength is varied by selecting one of peak wavelengths of the wavelength filter such as an etalon using the tunable filter (hereinafter referred to as second technique).